1. Field of the Invention
The present invention relates to an acquisition, pointing, and tracking scheme useful in a laser communication system.
2. Description of the Related Art
While free-space laser communication systems offer the potential to transmit data at very high data rates over long distances, developing practical systems that operate reliably in moving platform environments (e.g., airborne, space, and ground vehicles) presents unique challenges. Due to the changing positions of moving platforms, acquisition schemes and precise beam pointing and tracking capabilities are required to operate in these environments. Particularly with airborne platforms, where movement of aircraft can be rapid (compared to ground vehicles) and unpredictable (compared to satellites), it is critical that the pointing and tracking scheme provide accurate guidance for directing the data laser beams.
Consider a scenario in which two laser communication terminals whose relative positions may change are engaged in two-way communication (e.g., either one or both of the terminals are mobile). In each terminal, one option for determining the angular direction of the other terminal (i.e., the far-end terminal) is to split off a portion of the data laser beam received from the far-end terminal and determine the angle of arrival of the split-off data laser beam signal. This approach has a number of disadvantages. By using a portion of the received data laser beam for position detection, only the remaining portion of the received data laser beam is available for reception of the data, thereby reducing the signal power at the receiver and reducing the maximum operating range of the system. Moreover, it is desirable to minimize the beamwidth of the data laser beam in order to maximize signal strength and operating range. Given the limited angular extent of the data laser beam, initial acquisition of a remote terminal is difficult with the data laser beam. Likewise, once a communication link has been established between terminals, it may be difficult for the terminals to continuously track each other using narrow data laser beams, since either terminal can fairly quickly move out of the beam when the relative angular direction of the terminals is changing rapidly.
Further, current AC coupled position sensing detectors for determining the angle of arrival of laser beams have frequency bandwidths that extend up to only a few megahertz. In laser communications, data can be modulated onto the laser beam using modulation on the order of gigahertz up to hundreds of gigahertz. This modulation makes the data-carrying laser beams virtually invisible to AC coupled position sensing detectors, which currently don't have the bandwidth to sense the gigahertz modulation frequencies.
Additionally, laser tracking schemes can be susceptible to glint and solar radiation, and the track of a moving terminal may be lost if the terminal's line of sight approaches the line of sight of the sun or if persistent glint is present. Accordingly, there remains a need for improved schemes for acquisition, pointing, and tracking in laser communication systems, particularly in those involving moving platforms.